1. Field of the Invention
The present invention relates to antenna devices with primary radiators and openings, used for transmission in millimeter-wave bands. The invention also relates to communication apparatus and radar modules incorporating the antenna devices.
2. Description of the Related Art
In a conventional vehicle radar module utilizing a millimeter wave band or the like, a radar beam having high directivity is emitted in the forward and backward directions of the vehicle. Then, the radar module receives waves reflected by targets such as other vehicles running before and after the vehicle to detect distances from the targets and the relative speed of the vehicle with respect to the targets based on the time lag and the frequency difference between transmitted and received signals. In such a millimeter-wave radar module, when the angular range of detection is narrow, the beams of transmitted and received waves will be formed in fixed directions. However, when the angular range of the detection is wide and when a high gain needs to be maintained without deteriorating the resolution obtained in the detecting angular direction, the directions of the beams formed by the transmitted and received waves need to be changed while maintaining high beam directivities. Hereinafter, changing the beam directions will be referred to as beam scanning.
In an aperture antenna including a dielectric lens and a primary radiator, beam scanning is performed by changing the position of the primary radiator relatively with respect to the dielectric lens. As one example, there is known an antenna device described in (1) Japanese Unexamined Patent Application Publication No. 10-200331. In this case, as shown in FIG. 16, there is provided a single antenna device having a dielectric lens 25 and a primary radiator 1. The direction of a beam is changed by relatively changing the position of the primary radiator 1 with respect to the dielectric lens 25. In FIG. 16, the reference numerals 1a, 1b, and 1c simultaneously represent three positions of the single primary radiator obtained when beam scanning is performed. When the primary radiator is in the position 1a, a beam is formed as shown at Ba. When the primary radiator is in the position 1b, a beam is formed as shown at Bb. In addition, a beam as shown at Bc is formed when the primary radiator is in the position 1c. 
Furthermore, in (2) Japanese Unexamined Patent Application Publication No. 10-27299, there is described a vehicle radar module detecting objects by switching a plurality of antennas having different beam widths.
Besides, (3) Japanese Unexamined Patent Application Publication No. 10-142324 provides a radar module in which five reception beams are arranged in the beam-width range of a transmission antenna.
On the other hand, in the device (1), when the displacement of the primary radiator is increased in order to perform beam scanning over a wide angular range by using the single dielectric lens and the single primary radiator, the position of the primary radiator significantly deviates from the most suitable position for the dielectric lens and the gain of the antenna is reduced, thereby resulting in significant deterioration in the side-lobe level (characteristics). As a result, since the beam-scanning angle cannot be changed widely, scanning cannot be performed in a wide angular range. For example, since the beam cannot be oriented in a range over xc2x160xc2x0, it is difficult to detect objects over a wide range.
The radar module (2) has no function for detecting angular information on the direction of a beam. Thus, the directional information of an obstacle cannot be obtained. Additionally, there is a problem in that the number of antennas including primary radiators and lenses needs to coincide with the number of beams. Furthermore, the publication (2) describes only the concept of the module and does not clarify the realizing method.
In the radar module (3), the scanning angle is determined according to the adjustment between the direction of a beam emitted from the transmission antenna and the beam width of a reception antenna. Consequently, the wider the scanning angle, the broader the width of the transmission beam. However, it is difficult to greatly broaden the width of the transmission beam. Even if it can be broadened, that results in reduction in power density, whereby a detectable distance is reduced.
Accordingly, it is an object of the present invention to provide a high-gain antenna device capable of broadening the range of beam scanning and easily increasing the speed of scanning. It is another object of the invention to provide a radar module and a communication apparatus incorporating the antenna device, which have high detection capabilities.
According to a first aspect of the invention, there is provided an antenna device including a primary radiator arranged on a moving portion, a plurality of openings arranged on a fixed portion to receive electromagnetic waves radiated from the primary radiator to control the directivities of generated beams, and a unit for relatively displacing the moving portion with respect to the fixed portion to select each opening appropriate for primarily receiving each of the electromagnetic waves and to change the directions of the beams.
With this arrangement, even with the use of the single primary radiator, high-speed beam scanning can be performed over a wide angular range.
In addition, in this antenna, the plurality of openings may be formed by dielectric lenses. As a result, the entire structure of the antenna device can be simplified, thereby facilitating the design of the antenna device.
In addition, in this antenna, the openings may be formed by dielectric lenses and either reflectors or optical transmitters arranged between the dielectric lenses and the primary radiator. With this arrangement, the beam-scanning angle with respect to the displacement of the primary radiator can easily be broadened and the speed of scanning can be increased.
In addition, the antenna device may further include a unit for detecting the direction of the beam emitted from each of the openings. In other words, when beam scanning is performed with each of the plurality of openings, the direction (angular information) of each beam is detected. As a result, while using the plurality of openings, the beam can be oriented in an arbitrary direction.
In addition, the antenna device may further include a directional coupler formed by coupling a line arranged on the fixed portion to a line arranged on the moving portion and coupled to the primary radiator. This arrangement facilitates coupling between the line of the fixed portion and the line of the moving portion.
In addition, the lines arranged on the fixed portion and the moving portion may be nonradiative dielectric lines. As a result, signal transmission loss caused in a millimeter wave band can be reduced, and coupling with the primary radiator can be facilitated.
Furthermore, the degree of coupling between an input side and an output side in the directional coupler may be substantially 0 dB. As a result, insertion loss caused by the directional coupler between the line of the fixed portion and the line of the moving portion can be suppressed, thereby increasing output power.
Furthermore, the antenna device may further include shielding members arranged for shielding at least two predetermined openings from the rest of the plurality of openings. With this arrangement, even when the entire antenna device is made compact, electromagnetic waves from the primary radiator are emitted only to predetermined openings, selectively.
Furthermore, in this antenna device, a line connecting the centers of the openings may be not parallel to a direction in which the primary radiator is displaced so that the direction of the beam is three-dimensionally changed by linearly displacing the moving portion. This arrangement enables the three-dimensional beam scanning.
Furthermore, of the plurality of openings, the central opening may be larger than the remaining openings. With this arrangement, the width of a beam in the central direction is narrowed and the beam widths in directions away from the center are broadened.
Furthermore, in this antenna device, the dielectric lenses may be integrally formed over the plurality of openings. This arrangement facilitates the assembly of dielectric lenses and improves the directional accuracy of each dielectric lens.
According to a second aspect of the invention, there is provided a communication apparatus including the antenna device according to the first aspect, a transmission circuit for outputting a transmission signal to the antenna device, and a reception circuit for receiving a reception signal from the antenna device. This arrangement enables communications performing beam scanning over a wide angular range.
Furthermore, according to a third aspect of the invention, there is provided a radar module including the antenna device according to the first aspect and a unit for outputting a transmission signal to the antenna device and receiving a reception signal from the antenna device to detect an object reflecting electromagnetic waves sent from the antenna device. With this arrangement, high-speed detection of targeted objects can be performed over a wide angular range.
Furthermore, the radar module may further include a unit for controlling the displacement of the moving portion in such a manner that when the speed of a moving object incorporating the radar module is higher than a predetermined speed, the ratio of a time in which the electromagnetic wave radiated from the primary radiator is transmitted to an opening ready for a direction in which the moving object travels, of the plurality of openings, is greater than the ratio of a time in which the electromagnetic wave is transmitted to each of the remaining openings. With this arrangement, intensive detection can be made over a beam-scanning angular range according to the speed of the moving object.